The present invention is directed to improved acid electrolytes for depositing tin and tin-alloys on iron containing substrates. More specifically, the present invention is directed to improved acid electrolytes for depositing tin and tin-alloys on iron containing substrates which are self-fluxing.
Iron containing substrates such as strip steel may be electroplated with tin in large continuous plating machines at steel mills throughout the world. In such machines a large coil of steel sheet unwinds at one end of the machine and proceeds through cleaning and acid pickling stations followed by multiple tin electroplating stations to produce a tin deposit over the steel surface. The tin coating, as plated, exhibits a characteristic smooth matte surface.
The next section of the line is a variously known as the “flow-melting”, “flow-brightening”, or “reflow” section. The reflow operation is used to transform the matte deposit to the bright reflective finish typical of tinplate and to produce a thin iron-tin compound layer at the interface between the tin coating and the steel base, thereby improving corrosion resistance. The operation includes the steps of raising the temperature of the tin coating to above the melting point of tin, followed by immediate quenching to impart and achieve the desired properties of the deposit.
In the reflow operation, after the matte tinplate is rinsed, the steel sheet proceeds through a fluxing station. The term “flux” in this context refers to a substance that aids, induces, or otherwise actively participates in fusing or flowing. The application of flux is followed by drying and the reflow station itself which raises the temperature of the steel to slightly above the melting point of tin. The steel is then quickly quenched in water, resulting in a tin surface that has a bright finish. After reflow, the steel proceeds through other stations for treatments such as passivation, oiling and rewinding or cutting into sections at the exit-end of the machine.
A uniform, bright finish is achieved without blemishes or discontinuities if all of the above steps are optimally executed. A flux treatment prior to reflow is important to prevent formation of tin oxides or hydroxides. The formation of tin oxides and hydroxides may cause defects in the tin finish during reflow. This defect is observable on the surface of the tin as a white haze. Another common defect is a blue haze caused by acid etching of the tin. Many desirable tin electrolytes include acids such as phenolsulfonic acid, sulfuric acid, fluoborate and alkyl sulfonic acids. A common alkyl sulfonic acid used in tin electrolytes is methane sulfonic acid. However, when a sufficient quantity of methane sulfonic acid is present in the flux by contamination due to improper rinsing before the flux, it causes a blue haze effect. Typically, methane sulfonic acid in amounts of 0.8 g/L and greater cause the blue haze effect. For this reason, the rinsing steps prior to fluxing are critical to quality. In order to prevent blue-haze, it has been found that one needs to achieve greater than 95% rinsing efficiency in methane sulfonic acid based electrolytes.
In an attempt to address the problem of the formation of tin oxides and hydroxides and acid etching of tin, steel articles plated with tin are rinsed with water in counter-flow rinsing systems to dilute any tin electrolyte on the plated steel and to remove residual acid. Such a system typically includes a number of consecutive isolated tanks in which water is sprayed onto the strip. Between the tanks, rubber snubber rolls prevent water from being passed from one tank to the next. De-ionized (DI) water is fed into the last tank and the tank is allowed to cascade back into the previous tank, with the first tank cascading back into the electrolyte. In such a system, the strip is thus washed in increasingly cleaner water and an optimum of rinsing efficiency with minimal water consumption can be realized. Each stage can achieve about 60% removal, thus a two stage system can achieve 84% removal and a 3-stage 94%. The counter-flow rinsing systems also recover any tin electrolyte which is lost to the environment as dragout from the tin plated steel. Such dragout, which contains the electrolyte components, may present a hazard to the environment if not recovered. The tin, any additional metals, acids and other electrolyte components typically are environmentally unfriendly. Additionally, recovery of most of the electrolyte increases the efficiency and reduces the cost of the tin deposition process to the industry.
A typical system includes at least three dragout cells filled with counter-flowing water and the last drag-out cell would double in function as the flux cell. The phenol sulfonic acid based electrolytes, such as phenol sulfonic acid (PSA) itself, performs the function of a flux and additional PSA is usually added to the final dragout cell. PSA is thus dragged out into quench water and incurs waste-water treatment costs as not only is PSA carcinogenic, but it also has a high chemical oxygen demand (COD), a measure of its environmental impact.
As sulfuric, fluoborate and methane sulfonic acid based electrolytes are not self-fluxing, a separate fluxing agent needs to be employed. Examples of fluxing agents are hydrochloric acid, phenolsulfonic acids, or an acid salt such as ammonium chloride and zinc chloride. However, a number of the typically used fluxing agents have problems. Hydrochloric acid may cause hazing of tin deposits. Phenolsulfonic acids are pollutants which can not be discharged into the environment. Significantly, none of these fluxing agents are compatible with the tin electrolyte and thus the fluxing cell (or final dragout cell) has to be isolated from the rest of the electrolyte. The electroplating electrolyte itself is not compatible with the fluxing agent and thus in order to achieve a defect free reflowed surface, one needs to perform optimal rinsing with typically more than four counter-flow dragout cells in conjunction with a separate fluxing cell. Thus at least 5 cells (4 rinse, 1 flux) in addition to the electroplating cells need to be used.
Most lines are built with PSA-based chemistry in mind and usually have only two to three cells in addition to the electroplating cells. Thus, if any PSA line is to be converted to a more environmentally friendly electrolyte, additional cells are installed. Due to the limited footprint and extensive machinery present on such lines, inserting additional cells is not a trivial undertaking. For this reason, such conversions, despite strong economic and environmental drivers, are not commonplace.
U.S. Pat. No. 5,427,677 to Mosher discloses a flux for reflowing tinplate. The flux includes non-poisonous and environmentally friendly naphthalenesulfonic compounds, and excludes the undesirable phenolsulfonic acids. Acids which may be included in the flux are hydrochloric acid, sulfuric acid, citric acid, alkane sulfonic acids such as methanesulfonic acid, alkanol sulfonic acids and ammonium chloride. The flux is suitable for removing tin oxide and hydroxide and for preventing blue haze formation. The flux is also employed in a separate fluxing cell, isolated from the tin electroplating electrolyte.
Although there are tin electrolytes and fluxes which prevent tin oxide and hydroxide formation and prevent acid etching, there is still a need for improved tin compositions which address such problems.